Stress graded cable termination



Aug. 6, 1968 H- c. ANDERSON 3,396,231

STRESS GRADED CABLE TERMINATION Filed Jan. 18, 1967 JACKET SEMI-CONDUCTING LAYER SEMI FiOSEPUCTING 1 INVENTOR HARRY C ANDERSON United States Patent 3,396,231 STRESS GRADED CABLE TERMINATION Harry C. Anderson, Stratford, C0nn., assignor to General Electric Company, a corporation of New York Filed Jan. 18, 1967, Ser. No. 610,113 21 Claims. (Cl. 174-73) ABSTRACT OF THE DISCLOSURE Electric cable terminating means for substantially inhibiting ionization at the termini in which a semi-conductive coating is applied onto the insulation layer for a predetermined length from the high voltage output end to establish electrical contact with the shielding means. The coating has a predetermined resistance per square sufficient so that the electrical stress at the surface for said length does not exceed the ionization start level of the cable.

In a typical high voltage cable, a semi-conducting tape is wound around the metal conductor, and an insulation layer is extruded over this surface. A ground shielding means is then concentrically disposed over the insulation, which usually comprises a semi-conducting layer and a metallic return shield. The semi-conducting layer, for example, may be a nylon tape impregnated with carbon black, or may be polyethylene or butyl rubber having incorporated therein carbon black. The metallic return shield for returning current may be copper, or tinned copper, wrapped around the semi-conducting layer or may be a copper braid concentrically disposed over said semiconducting layer. The structure may be further enclosed by a jacketing material such as a polyvinyl chloride layer or a metallic jacket. In the cable construction, it is important to eliminate or minimize any voids, such as in the insulation or at the interfaces, which potentially are a source of breakdown. That is, under high voltage conditions encountered, the voids may ionize thereby leading to the eventual breakdown of the cable.

Cable is tested for voids by an ionization level test. According to this test, at each terminal of the cable the ground shielding means is stripped back to expose the insulation layer. The edge of the ground shielding means is cut uniformly and carefully to avoid nicking the insulation. The raw edge of the ground shielding means is then taped to provide a tight fit to the insulation. There should be no gaps between the ground shielding means and insulation, and there should be no protruding points or tips extending from the ground shielding means. A cup containing mercury, insulated from ground, is immersed into a tank of oil, and one or both terminals are then inserted into the mercury. Voltage is applied through the mercury cup to the cable, and the voltage is increased until ionization occurs. The voltage level at which ionization occurs in the cable coincides with the visual display on an oscilloscope or other suitable instrument.

One distinct disadvantage with the oil termination is that the semi-conducting layer is attacked or dissolved by the oil thereby releasing the carbon black, or other conductive component. Consequently, the contaminated oil conducts current which gives a false ionization reading. This will be interpreted as a cable failure when in fact it may be a terminal failure.

This invention has as its purpose to provide an electric cable terminating means substantially eliminating or inhibiting ionizaton at the terminus, while overcoming the disadvantages of the prior art.

In an insulated cable such as of the type described above, at or near the termini a high voltage concentration exists along the edge of the grounded shield which results in ionization or corona discharge. It is known that this 3,396,231 Patented Aug. 6, 1968 lCC ionization may be reduced or minimized by applying a semi-conductive coating onto the grounded shield and over a portion of the insulation layer. The coating has a non-linear current characteristic, and a non-linear capacitive current is introduced along the coated portion. As a result, a voltage distribution occurs which results in a substantially linear voltage drop along the insulation layer thereby diminishing the stress concentration near the grounded shield. Hence, the electric field at the termination of the cable is rendered more uniform thereby reducing or substantially eliminating ionization at the terminal. According to my invention, a linear resistive current is introduced to the environment which is sufliciently larger in magnitude than the capacitive current to overmask the latter and thereby approach a substantially linear current.

The invention, together with its objects and advantages, will best be understood by referring to the following detailed specification, and to the accompanying drawings, in which:

FIGURE 1 is a perspective view of a cable of typical construction with portions thereof cut away for the purpose of better illustrating its construction;

FIGURE 2 is a side elevational view of a cable showing a terminating means falling within the scope of this invention;

FIGURE 3 is a front elevational view of the cable of FIGURE 2; and

FIGURE 4 illustrates another embodiment of the invention.

In a broad aspect of the invention, I provide cable terminating means characterized by substantially no ionization by applying a semi-conductive coating onto the insulation layer of the cable from the high voltage output end at the terminus to the ground shielding means. In this manner, electrical contact is established between the high voltage output end, i.e. conductor, and the shielding means. The coating extends over the insulation of the cable for a predetermined distance or length, sometimes referred to herein as the termination length and explained hereinbelow in greater detail. The coating material is characterized by a resistance per square which value is predetermined such that the electrical stress or transverse voltage gradient at any point on the termination does not exceed the ionization level of the cable. It will be observed that a non-linear capacitive current is established caused by the capacitive effect of the semi-conductive coating on the insulation layer. However, the coating extends from the high voltage output end to the ground shielding means thereby establishing a resistive current as a second current to the environment on the outside of the insulation where the non-linear voltage stress occurs that causes ionization at the terminus. The resistive current is linear, and, by design, is substantially larger in magnitude than the capacitive current. When both currents are added together, the resistive current over-masks the capacitive current, and consequently approaches a substantially linear current effect. As a consequence, a uniform voltage drop is established along the coated portion of the cable termination from the ground shielding means to the high voltage output end thereby substantially eliminating ionization in the cable termination.

Referring to the drawings wherein like reference numerals designatesimilar parts throughout, there is shown a coaxial cable of typical construction indicated generally by the numeral 10, such as might be adaptable for carrying a voltage load of 5 to 15 kilovolts, or higher. The cable includes an inner metallic conductor 12 illustrated in the form of a stranded cable, although it should be understood that the conductor 12 may comprise a solid conductor. Generally, a semi-conducting layer 14, e.g.

tape, is applied around the metal stranded conductor for the purpose of establishing a good electrical cOntact between the conductor and the insulation and further to shield out stresses thereby equalizing all stresses of the individual strands. The metal conductor, with a semiconducting tape wound thereon, is surrounded by a relatively thick insulating layer 16 which is usually applied by extrusion. The insulating material is typically a thermosetting plastic such as cross-linked polyethylene or ethylene-propylene rubber, which may be filled with mineral clay or other suitable fillers. Also, the cable includes a ground shielding means comprising semi-conducting layer or tape 18 and a metallic return shield 20, and, overlying this, is outer jacket 22 made of conventional material such as polyvinyl chloride.

FIGURE 2 shows a terminating means prepared in accordance with the invention for an ionization level test. Outer jacket 22 is first stripped from the cable termination for a certain distance. The amount stripped will depend upon the termination length required, as explained hereinbelow, but there is no need to strip from the cable more than an inch or two of the jacket beyond the termination length. The copper shield or tape 20 is then unwound slightly more than the full distance of the termination length to expose the semi-conducting layer, and, for ionization testing, the end of the tape is connected to ground. Next, the semi-conducting layer 18 is removed substantially the full termination length leaving exposed insulation layer 16. As explained hereinbelow, this length typically is in the range of 6 to 15 inches. A small portion of the conductor 12 extends beyond the marginal edge of the insulation layer, and the semi-conducting tape 14 is stripped from the exposed end of the conductor.

The termination length is then cleaned of dirt, grease, oil or other contaminants as by washing the termination with Vythene, carbon tetrachloride or other suitable solvent. After the solvent has dried, a semi-conductive coating material 24, described in greater detail hereinafter, is applied to the termination length. Coating 24 extends from the high voltage output end, i.e. conductor, to the ground shielding means to establish electrical contact with the shielding means. The coating is applied continuously over the circumference of the insulating layer 16 and desirably about inch over the circumference of the semi-conducting layer 18 and about A inch over the conductor 12. The coating may be applied by painting with a brush, spraying, dipping or any other suitable means, and is then permitted to dry as in air.

In conducting the ionization level test, metallic shield 20 is connected to ground, and a cable lug, which is ionization free, is connected to the metal conductor at each terminus. At least one cable lug is connected electrically to the test equipment. Each cable, depending upon its class and size, must pass etablished standards with regards to ionization level. In a typical test procedure, voltage is applied to a cable to a high potential level as required by the standard, held there for minutes, and then lowered gradually. If ionization occurs as observed on an oscilloscope or other suitable test apparatus, the voltage is lowered until it is found at what voltage ionization is extinguished. If this occurrence of ionization is at a potential above the required minimum, the cable is passed as satisfactory. Because of my invention, ionization is substantially eliminated at the terminals, and any ionization detected is therefore in the cable.

In accordance with the invention, ionization or corona discharge is substantially precluded by grading the electrical stress sufficiently to maintain the stress along the termination length below the ionization start level of air. The ionization start level, regardless of cable size, can be found from Paschens law. For example, at about inches from the cable termination the ionization start level is about 46 volts per mil (voltage calculated at root mean square), and therefore the total electrical stress along the termination length should be less than 46 volts per mil. The

ionization start level depends on atmospheric conditions, e.g. humidity, temperature and barometric pressure, and therefore may vary by as much as about 3 volts per mil. The resistance electrical stress may be calculated from (1) the voltage load for which the cable is constructed to carry, (2) the circumference of the insulation, (3) termination length, and (4) the resistance per square of the coating material.

In calculating the resistance electrical stress, the load bearing characteristic and circumference of the insulation are set by the cable undergoing testing. Generally, the invention is applicable to cable adaptable to carry a voltage of from about 5,000 to 15,000 volts, but cable with higher load bearing characteristics is also applicable. The circumference may vary depending on such factors as the type of insulation used, conductor size, and the like, and generally may have an insulation circumference ranging from about one inch to 5.5 inches. For example, a typical 15 kilovolt co-axial A.C. cable carrying a current of about amperes or higher, constructed as shown in FIGURE 1 and having a mineral filled cross-linked polyethylene insulating layer and polyvinyl chloride jacket, may have a circumference of about 1.8 inches.

The termination length may vary depending largely upon the cable size and voltage load bearing characteristic. As the length of the termination increases, it is necessary to increase the resistive current in order to achieve a linear voltage drop. The heat generated is proportional to the square of the current, and therefore a small increase in current can result in high heat losses. If the termination becomes too hot, arcing will occur between the metal conductor and metal shield which will short out the test equipment, i.e., high potential transformer. On the other hand, if the termination is too short, arcing will occur through air between the conductor and metal shield. For conventional high-power cables, such as cable adaptable for carrying high voltage loads of from about 5 to 15 kilovolts, the termination length typically is from about 6 to 15 inches, but may be more or less depending on such factors as voltage load and circumference. However, the termination length may be determined experimentally by one skilled in the art for each production specification of cable.

The semi-conductive coating is characterized by a resistance per square. Typical coating materials useful for this invention include ultra-high resistance paint comprising carbon black filler and an inert modified styrene resin as binder, and having a resistance per square of 10 to 10,000 megohms per square. The useful range for the resistance value of the coating materials may vary depending primarily on cable configuration, voltage and thermal conductivity of the insulation. In determining this range, cable configuration factors include the diameter and/ or circumference of the metal conductor, the relation of the diameter across the insulation layer to the diameter of the metal conductor, and the mass of the insulation layer and metal conductor. The thermal conductivity of the insulation layer is significant in that the more conductive the insulation, the more heat will be absorbed by the conductor, which acts as a heat sink, and consequently a coating material with a lower resistance value can be used. I have found, for example, for a 15 kilovolt, N0. 2 stranded AWG, having a circumference of 2.09 inches, and employing a 10 inch termination length, that a coating material having a resistance of as low as 10 megohms per square may be used. If the resistance per square is too low for the cable excess heating occurs causing a flash over. On the other hand, if the resistance per square is too high, then the capactive current causes the voltage gradient to become sufficiently non-linear and ionization along the termination can result. In the event the electrical stress or transverse voltage gradient along the termination length exceeds the ionization level of the cable, a coating with a lower resistance per square is employed. Generally, for terminating means falling within the scope of this invention, the electrical stress for typically high voltage cable should be less than about 46 volts per mil, and for purposes of a safety factor I have found a stress of about 30 volts per mil to be particularly desirable. It should be understood, however, that in determining the electrical stress, atmospheric conditions must be taken into consideration (as explained above), and therefore this value may vary somewhat. In this range, a coating having a resistance of about 100 to 300 megohms per square is specially useful.

To illustrate the difference between a cable not having its terminations protected and cable with terminations prepared in accordance with the teaching of this invention, 15 kilovolt co-axial, ungrounded power cables were tested. This cable was constructed substantially as shown in FIG- URE 1, and was provided with a 2/0 AWG copper conductor, a mineral filled cross-linked polyethylene insulating layer, a butyl rubber-carbon black filled semi-conducting layer, copper tape return shield and a polyvinyl chloride jacket. The insulation had a circumference of about 2.72 inches.

The cable not having its termination length coated or covered was tested in the oil tank as explained above, and 15 reels of this cable showed an average ionization start level of 17.6 kilovolts. On 19 reels of cable, a semiconductive paint manufactured by Micro Circuit Company and having a resistance per square of 100' megohms was painted on termination lengths measuring inches. The ionization start levels averaged 19.5 kilovolts. This illustrates clearly the superior results obtained in precluding ionization or corona discharge at the terminations, and that cable can be tested at higher voltage levels by reason of the invention without ionization occurring in the terminations.

To further illustrate the invention, 6 cable specimens of a kilovolt No. 2 stranded AWG were prepared in accordance with the invention. Tests results are shown in the following table.

TABLE.IONIZATION LEVEL FOR 15 KV. CABLE At the ionization levels set forth in the table above, no ionization occurred showing that ionization was precluded at the terminations. For the test using a coating material having 10 megohms per square, the termination at the end of the test was warm thereby indicating for this particular cable that heat losses were being induced and it would not be advisable to use a coating with any lower resistance per square.

Where desired, the termination may be further protected upon long exposure to high voltage and other environmental conditions by covering the semi-conductive coating with an insulating film or coating. This embodiment is illustrated in FIGURE 4. This insulating film 26 protects the semi-conductive coating from the air and humidity and further protects the semi-conductive coating from spalling. Suitable protective insulating films include, for example, electrical insulating tape such as polyvinyl chloride tape or Irrathene or SPT tape, or an organic or resinous film such as an epoxy resin. To illustrate the value of this embodiment of the invention, the terminals of a 15 kilovolt cable, such as the one described in the first example above, was painted with a semi-conducti-ve paint having a resistance per square of 100 megohms. The cable was operated at 150% of rated voltage thereby resulting in an applied voltage of 13 kilovolts, and was cycled three times per day with sufiicient current to raise the temperature of the cable to about 90 C. This cable was on load cycle for 26 days and showed no indication of terminal breakdown due to ionization or corona discharge.

I claim:

1. Electric cable terminating means for substantially inhibiting ionization at the termini of said cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, the improvement which comprises: a semi-conductive coating having a substantially dominate linear current characteristic upon application of voltage on said insulation layer extending for a predetermined length from and in contact with the high voltage output end at each terminus to said shielding means to establish electrical contact with said shielding means, said coating having a predetermined resistance per square suflicient so that the voltage gradient at the surface for said length does not exceed the ionization level of the coated termini of the cable.

2. Electric cable according to claim 1 wherein said coating has a resistance of not less than about 10 megohms per square.

3. Electric cable according to claim 1 wherein said predetermined length is from about 6 to 15 inches.

4. Electric cable according to claim 1 wherein the electrical stress along said predetermined length is less than 46 volts per mil.

5. Electric cable according to claim 1 including a protective coating covering said semi-conductive coating.

6. Electric cable terminating means of substantially inhibiting ionization at the termini of said cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, the improvement which comprises: a semi-conductive coating having a substantially dominate linear current characteris upon application of voltage on said insulation layer extending for a length of from about 6 to 15 inches from and in contact with the high voltage output end at each terminus to said shielding means to establish electrical contact with said shielding means, said coating having a resistance ranging from about to 300 megohms per square whereby the electrical stress along said coated length is less than 46 volts per mil.

7. Electric cable according to claim 6 including a protective coating covering said semi-conductive coating.

8. A method for determining ionization in a cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, which comprises: applying a semioonductive coating having a substantially dominate linear current characteristic upon application of voltage for a predetermined length on said insulation layer from and in contact with the high voltage output end at each terminus to said shielding means to establish electrical contact with said shielding means, said semi-conductive coating having a predetermined resistance per square suflicient so that the voltage gradient at the surface for said length does not exceed the ionization level of the coated termini of the cable, and subsequently applying voltage to said cable to measure ionization in said cable.

9. A method according to claim 8 wherein said coating has a resistance of not less than about 10 megohms per square.

10. A method according to claim 8 wherein said predetermined length is from about '6 to 15 inches.

11. A method according to claim 8 wherein the electrical stress along said predetermined length is less than 46 volts per mil.

12. A method according to claim 8 including a protective coating covering said semi-conductive coating.

13. A method for determining ionization in a cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, which comprises: applying a semioonductive coating having a substantially dominate linear current characteristic upon application of voltage for a length ranging from about 6 to 15 inches on said insulation layer from and in contact with the high voltage output end at each terminus to said shielding means to establish electrical contact with said shielding means, said semiconductive coating having a resistance of 100 to 3 megohms per square whereby the electrical stress along said coated length is less than 46 volts per mil, and subsequently applying voltage to said cable to measure ionization in said cable.

14. A method according to claim 13 including a protective coating covering said semi-conductive coating.

15. A method for substantially inhibiting ionization at a terminus of an electric cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, which comprises: applying a semi-conductive coating having a substantially dominate linear current characteristic upon application of voltage for a predetermined length on said insulation layer from and in contact with the high voltage output end at said terminus to said shielding means to establish electrical contact with said shielding means, said semi-conductive coating having a predetermined resistance per square sufiicient so that the voltage gradient at the surface for said length does not exceed the ionization level of the coated termini of the cable.

16. A method according to claim 15 wherein said coating has a resistance of not less than about megohms per square.

17. A method according to claim wherein said predetermined length is from about 6 to 15 inches.

18. A method according to claim 15 wherein the electrical stress along said predetermined length is less than 46 volts per mil.

19. A method according to claim 15 including a protective coating covering said semi-conductive coating.

20. A method for substantially inhibiting ionization at a terminus of an electric cable comprising an insulation layer surrounding a conductor and a ground shielding means concentrically disposed over said insulation layer, which comprises: applying a semi-conductive coating having a substantially dominate linear current characteristic upon application of voltage for a length ranging from about 6 to 15 inches on said insulation layer from and in contact with the high voltage output end at said terminus to said shielding means to establish electrical contact with said shielding means, said semi-conductive coating having a resistance of about to 300 megohms per square whereby the electrical stress along said coated length is less than 46 volts per mil.

21. A method according to claim 20 including a protective coating covering said semi-conductive coating.

References Cited UNITED STATES PATENTS 3,210,461 10/1965 Berg et al. 174-127 3,349,164 10/1967 Wyatt 17473 3,015,774 1/1962 Eigen 32454 3,210,460 10/ 1965 Suelmann 17473 3,246,237 4/1966 Mole 324-54 OTHER REFERENCES Virsberg et al., article entitled, A New Termination for Underground Distribution, presented at IEEE Summer Power Meeting, New Orleans, La., July l0l5, 1966, Paper No. 31PP66-52l.

LARAMIE E. ASKIN, Primary Examiner. 

